Shower faucet without plaster ground

I am installing a shower faucet (moen L2353). The plaster ground was not installed behind the shower wall and there is limited access. How can I attach the cover without that plastic piece installed properly. The hole through the shower wall is approx. 3 inches.

The plaster guard is not designed to stay on the rough-in valve...so it should not have any impact on installing the trim. BUT, if the hole through the wall was not at least as big as the plaster guard, there may be no way to get to the holes (which are in the valve, not the plaster guard) to actually attach it. the plaster guard is there to help position the valve properly in the wall and to ensure the hole is big enough and in the right place so you CAN install the trim.

The only time the plaster guard is designed to stay in place is to help stiffen things up when you have a thin-wall installation (like a thin fiberglass shower surround). And even then, if the hole was sized and positioned properly, it would not affect the ability of the trim to be installed.

You may need to try to cut or trim the tile on the wall so that the screw holes are uncovered.

The plaster guard does indeed serve a use. There is a gasket on the back of the circular chrome trim which needs to butt up against something firm
to create a good seal. Without the firm fit, the gasket will sort of work, but at some point in the future (22 years for one of my Moen valves),
the gasket comes loose from the trim, and water leaks behind the valve (down to the ceiling in the room below, in my case!).

I jerry-rigged a plaster guard substitute out of a circular piece of stiff plastic; am installing this afternoon.

quote; The plaster guard does indeed serve a use. There is a gasket on the back of the circular chrome trim which needs to butt up against something firm
to create a good seal.

EVERY plaster ground specifies that it is removed before installing the trim, with one exception. When you have a fiberglass tub/shower wall, SOMETIMES the opening in the wall can be made undersized, then the plaster ground it placed behind the wall and the trim is installed clamping the wall between the ground and the trim. OTHERWISE, the wall is what the "rubber gasket" presses against. Th plaster ground is supposed to be installed "flush" with the finished wall. If the plaster ground had been installed "behind" the shower wall the valve would probably be too deep to install the trim without an extension kit.